1. Technical Field
This invention relates to circuits, and the operation thereof, for driving brushless D.C. motors, and more particularly to such a circuit that provides for faster response of a brushless D.C. motor during acceleration and deceleration.
2. Background Art
The conventional control of brushless D.C. motors follows the following general pattern:
The A.C. power mains (one-phase or three-phase being the most common) are supplied to a full-wave rectifier with capacitor filtering. The filtered full-wave-rectified output of this operation is essentially a constant voltage D.C. supply with the output voltage varying only as the power mains fluctuate. The filtered D.C. power is supplied to a pulse-width modulator which provides a D.C. output having a voltage which may be varied from "zero" to the full D.C. bus voltage entering the pulse-width modulator from the filtered D.C. power supply. The output voltage of the pulse-width modulator is varied by varying the on-time to off-time ratio, or the "duty cycle", of a switching circuit internal to the pulse-width modulator. Control of this duty cycle is accomplished by comparing speed information feedback from a tachometer, connected to the motor, to a speed set point. The voltage output of the pulse-width modulator is therefore adjusted automatically in the pulse-width modulator to be in equilibrium when the motor speed comports with the set speed.
The voltage output of the pulse-width modulator is supplied to a polyphase switching circuit, which commutates the windings of the motor in concert with the motor shaft position through information from a motor shaft position encoder so that the appropriate windings are energized in accordance with what the shaft position dictates.
The motor may or may not have a current control operating mode. If it does, current information from the motor is fed back to a current limit circuit, which controls chopping in the polyphase switching circuit.
A major disadvantage of the conventional control scheme described above is that the pulse-width modulator has a time constant inherent in its design, which prevents rapid response to command changes. This means that the voltage output from the pulse-width modulator cannot rise suddenly to cause the motor to accelerate rapidly to a higher speed when an acceleration command is given. A similar lag is experienced when a deceleration command is given. Furthermore, the current rise time during acceleration and deceleration is limited to the current rise time determined by the rate of change of the pulse-width modulator output voltage as governed by speed.